1. Field of the Invention
The present invention is relates to a method of manufacturing a printed circuit board, and particularly a method of precisely forming a circuit pattern of a conductive material on a three-dimensionally molded substrate.
2. Disclosure of the Prior Art
For manufacturing a printed circuit board which is provided with a circuit pattern of a conductive material on an irregular surface of a three-dimensionally molded substrate, it is difficult to directly adopt the prior art's methods of forming a circuit pattern on a flat surface of a substrate. Therefore, for replying the demands, a variety of unique methods of manufacturing such a printed circuit board have been proposed.
For example, a Japanese Patent Early Publication [KOKAI] No. 1-298792 discloses a method of manufacturing a printed circuit board. A metallic layer is formed on an irregular surface of a three-dimensionally molded substrate, and also a photoresist layer is formed on the metallic layer. A cubical photomask is formed with a body made of a light-transmissive material, a replica surface fitted to the irregular surface, and a mask pattern described on the replica surface. The photomask is placed on the substrate such that the replica surface is precisely fitted to the irregular surface of the substrate. A light beam is radiated to the photoresist layer through the photomask to obtain an exposed pattern of the photoresist layer. The photoresist layer of the exposed pattern is removed to leave a first area of the metallic layer without the photoresist layer. An additional metallic layer is formed on the first area of the metallic layer by an electrodeposition or electroless deposition technique. Subsequently, the photoresist layer of an non-exposed pattern is removed to leave a second area of the metallic layer. The metallic layer of the second area is etched to form the circuit pattern of the metallic layer corresponding to the first area on the irregular surface of the substrate.
However, the cubical photomask having such a replica surface needs an expensive production cost and a lot of making steps thereof. In addition, since the photomask is formed by casting, for example, a silicon resin into the irregular surface of the substrate and hardening the resin, it would be difficult to precisely form the replica surface of the photomask as the irregular surface of the substrate becomes more complex.
On the other hand, U.S. Pat. No. 5,168,624 discloses another method of manufacturing a printed circuit board comprising the following steps. A conductive layer 20w is formed on an irregular surface of a three-dimensionally molded substrate 10w, and also an electrodeposited resist 30w is formed on the conductive layer 20w. A parallel light beam is radiated to a photomask 40w placed in front of the electrodeposited resist 30w to obtain an exposed pattern of the electrodeposited resist 30w, as shown in FIG. 22. The photomask 40w is formed with a mask pattern 41w and a light-transmissive pattern 42w. After the electrodeposited resist 30w of the exposed pattern is removed to leave a patterned area of the conductive layer 20w without the electrodeposited resist 30w, the conductive layer 20w of the patterned area is etched to form a circuit pattern of the conductive layer 20w corresponding to a non-exposed pattern of the electrodeposited resist 30w on the irregular surface of the substrate 10w.
However, when the parallel light beam is radiated to the irregular surface through the photomask 40w, there causes a problem of diffraction of the parallel light beam at the light-transmissive pattern 42w, which becomes more pronounced as a distance "D" between the light-transmissive pattern 42w and the electrodeposited resist 30w increases. That is, a width of the electrodeposited resist 30w exposed to the parallel light beam passing through a first light-transmissive pattern 43w which substantially contacts with the electrodeposited resist 30w, that is, D1.perspectiveto.0, would be equal to a width of the first light-transmissive pattern 43w. However, a width of the electrodeposited resist 30w exposed to the parallel light beam passing through a second light-transmissive pattern 44w which is spaced away from the electrodeposited resist 30w by a distance "D2", that is, D2&gt;0, becomes wider than a width of the second light-transmissive pattern 44w because of the influence of diffraction. Therefore, when the three-dimensionally molded substrate 10w has a considerable irregular surface, it would be difficult to precisely form on the irregular surface a circuit pattern of the conductive layer 20w.